Refinery Based Case Study of a Novel Gasket Designed for Use in Problematic Shell and Tube Heat...

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1 Abstract

This paper provides a technical overview of a novel gasket design engineered toaddress sealing issues associated with the operation of shell and tube heatexchangers. It discusses field service experience using the gasket on a UK refinery.

Shell and tube heat exchangers are commonly used items of process equipment inmany industries. Depending on how they are operated heat exchangers can presentsignificant technical challenges with regard to achieving leak free sealingperformance. Fluctuations in axial load and radial shear effects brought about bythermal transients around the tube sheet girth flanges during start up or normaloperation can a have a significant effect on seal performance. Additional factors suchas available gasket load imposed by exchanger design; available space, thepresence of stress raising nubbins and ease of access and installation can also havea bearing on gasket design.

Refinery processes involving shell and tube heat exchangers can be particularlydemanding, equipment is often old and can involve elevated temperatures, highpressures and cyclic operation. It is said that every refinery has its problemexchangers. Additional requirements for improved asset efficiency and emissionscompliance place ever increasing demands on gasket performance.

The novel gasket design offers benefits over traditional gasket styles morecommonly used in shell and tube heat exchanger applications. It offers superior longterm sealing performance, particularly in bolted connections subject to thermaltransients.

Refinery field service discussed in the paper reviews sealing performance in anumber of problematic exchangers at the tube sheet to channel and tube sheet to

shell connections. The heat exchangers, used to process heavy residual

Refinery Based Case Study of a Novel Gasket Designed for Use inProblematic Shell and Tube Heat Exchangers

B.Sc. Dene Halkyard

Flexitallic Ltd, Cleckheaton, United Kingdom



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hydrocarbons are in excess of 30 years old with a history of managed leakage. High

temperature, cyclic operation, seasonal fluctuations in product demand and the

requirement for regular tube cleansing exacerbates the potential for seal failure.

2 Background

As an international manufacturer of industrial seals and gaskets Flexitallic has been

manufacturing and supplying girth, tube-sheet and floating head gaskets for use on

shell and tube heat exchangers for many years. As a consequence Flexitallic

Applications Engineers are familiar with the apparent idiosyncrasies associated with

the sealing performance of individual exchangers. Shell and tube heat exchangers

can come in many designs and is a subject beyond the scope of this paper, however,

they are broadly categorised on the basis of service or construction. See ref. [1] for

additional information on this topic. All shell and tube heat exchangers contain the

following main elements:

• Shell

• Shell cover

• Tubes

• Channel

• Channel cover

• Tube-sheet

• Baffles

• Nozzles

A schematic of a sectional view of typical shell and tube heat exchanger with a

removable channel cover and stationary tube-sheet, floating head with backing ring

and removable shell cover is shown in figure 1.0. The location of girth, tube-sheet

and floating head gaskets is indicated in red and by the presence of a circled red

number.

Figure 1.0: Heat Exchanger Schematic

Channel cover

Channel

Shell

Shell cover

Tubes

Tube-sheet

Nozzle

Baffles

Floating head

1235

4



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Gasket positions indicated by the presence of red circled numbers are generally

named in accordance with the following notation:

• Location 1 – Channel cover or lid gasket

• Location 2 – Channel to tube-sheet gasket

• Location 3 – Shell to tube-sheet gasket

• Location 4 – Floating head gasket

• Location 5 – Shell cover gasket

Nozzles are pipe connections that are used to connect the exchanger to process

piping consequently they utilise standard flanges and gaskets such as those defined

in ASME B16.5 and ASME B16.20 respectively. However girth i.e. channel cover,

tube-sheet, shell cover and floating head connections are usually dimensionallybespoke and gaskets are designed accordingly. Flange face configuration of

bespoke connections is generally recess to spigot (male to female) in line with TEMA

guidelines (see ref. [1]). In spigot to recess face configurations the gasket, when in-

situ, is confined on the outer diameter. This kind of arrangement can give rise to

gasket width limitations particularly on floating head connections that can have an

impact on gasket selection.

For multi-pass exchangers the direction or channelling of process fluid is controlled

by the use of partition plates on the tube side of the exchanger. This necessitates the

use of gaskets with the incorporation of pass bars. The number and location of passbars is dictated by the direction and complexity of the process flow on the tube side

of the exchanger. The pressure differential across the pass bars is usually

significantly lower than that between the shell and tube side of the exchanger or

between process media and the atmosphere. Depending of gasket style pass bars

can be integrated i.e. part of the primary seal, or fixed in position by welding.

An area of particular interest is the gasketed connections located across the tube-

sheet. The tube-sheet is the barrier that separates the two process fluids i.e. the

shell and the tube fluids, between which heat transfer takes place. As a

consequence gaskets at these locations can be, depending on operational

conditions, subject to changes in both axial and radial load. In a fixed tube-sheet

arrangement, as shown in figure 1, there is a gasket located on both the shell and

channel side of the tube-sheet. In such cases it is generally recommended to use the

same gasket style in both locations. It is regular practice to use common bolting

across the tube-sheet to effect compression of both the shell to tube-sheet and

channel to tube-sheet gaskets. A schematic of this arrangement is shown in figure 2.



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Figure 2: Tube-sheet - Bolting and Gasket Arrangement

3 Current Gasket Technology

Various styles of gasket are used in shell and tube heat exchangers. The following

short overview represents the most commonly used styles encountered in the Oil

and Gas industry.

3.1 Metal Jacket Gaskets

The traditional gasket style used for the sealing of non-standard heat exchangerconnections in the Oil and Gas Industry has for many years been based around

metal jacketed technology. Metal jacketed gaskets, as the name implies, are

comprised of a thin metal jacket, typically 0.4mm formed around a soft compressible

core. Common jacket materials include soft iron, carbon and austenitic stainless

steels in combination with core materials such as graphite foil, millboards and fibre

reinforced sheet gasket materials. Figure 3 shows a typical section through a double

metal jacketed gasket.

Figure 3: Double Metal Jacketed Gasket - Section

Shell/tube-sheet gasket

Channel flangeShell flange

Bolting

Channel/tube-sheet gasket

Metal jacket

Core

Tube sheet

Box

Lid



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Other metal jacketed styles are sometimes used incorporating surface corrugations,

internal metal components or of single jacket construction. Metal jacketed gaskets

have several inherent weaknesses with regard to creating and maintaining a sealnamely; they require a relatively high unit stress to form a seal because of the

requirement for metal deformation (stress raising nubbins are sometimes used to

mitigate this). Successful metal to metal sealing also requires relatively smooth and

blemish free flange sealing faces, this can be particularly problematic on older

equipment. They possess very poor recovery characteristics; as a consequence

sealing performance under load fluctuations can be compromised. Resistance to

radial shear effects has been shown to be poor under RAdial Shear Tightness

(RAST) testing.

3.2 Corrugated Metal Gasket

Corrugated metal gaskets (CMG) are also used in used in heat exchanger

applications. They are comprised of a relatively thin metal, typically 0.4 mm thick,

core that has been concentrically corrugated and faced with a soft facing material.

Corrugation pitch and height can vary depending on the manufacturer and gasket

geometry. A typical material combination would be austenitic stainless steel in

combination with a graphite based facing material. See figure. 4.

Figure 4: Corrugated Metal Gasket – Section

This style of gasket offers some benefit over double jacket gaskets with regard toreduced seating stress and flange surface requirements. It is generally not suitable

for use in narrow land width sealing applications and may not be suitable for cyclic

service because of resilience limitations.

3.3 Spiral Wound Gaskets

Spiral wound gaskets (SWG) have been used in heat exchanger service for many

years and special TEMA type designs have been developed to assist in gasket

Installation. SWG are manufactured by spirally winding alternative wraps of a thin

strip of a metal (wire) and non-metallic sealing material (filler) around a mandrel. Thestrips are formed into a chevron profile during the winding process, resistance spot

welding the wire at the start and termination of the winding process results in a

Metal core

Facing material



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sealing element. Gasket resilience and stiffness can be altered to suit the specific

application. Handling and installation of larger gaskets can present difficulties and

care may need to be exercised to prevent the gasket from ‘springing’ if twisted ormishandled. Over compression of the sealing element is normally prevented by the

incorporation of a solid metallic inner compression ring of predefined thickness. The

requirement for compression control can limit the use of SWG, especially in

applications where space is limited, such as floating head connections. Typical

materials of construction used in hydrocarbon service are graphite in combination

with austenitic stainless steel. SWG offer significant advantages over metal jacketed

gaskets in several key areas. Namely, lower seating stress requirements, improved

resistance to radial shear effects and an increased tolerance to sealing face surface

finish. Figure 5 shows a section through a spiral wound gasket modified for heat

exchanger service.

Figure 5: Spiral Wound Gasket - Section

3.4 Kammprofile Gaskets

Kammprofile gaskets are comprised of a serrated solid metal core with soft non-

metallic facing material bonded to each face. Serration profile, facing material

thickness and density should be carefully controlled as it can have a bearing on

sealing performance. As with SWG, for hydrocarbon service typical materials of

construction are stainless steel and flexible graphite. Benefits offered over other

gasket styles are low stress sealing, their ability to accommodate flange face defects,

use of narrow sealing widths, high load bearing capability and ease of handling andinstallation - even for very large gaskets. A typical schematic of a sectioned

kammprofile gasket is shown in figure 6.

Figure 6: Kammprofile Gasket - Section

Inner compression ring Outer location nose

Metal core

Facing material

Sealing element

Serrations



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4 Novel Gasket Design

4.1 Design and Construction

Over many years of field service experience the shortcomings of traditional gasket

styles in problematic heat exchanger applications is well known to Flexitallic

engineers. With the objective to address such issues, extensive research and

development work was undertaken that resulted in the production of a novel gasket,

herein known as the Change gasket. The Change gasket has been designed to

possess the optimum combination of both stiffness and resilience and as a

consequence be capable of effecting a high performance seal under conditions of

long term cyclic service; such as those routinely encountered in demanding shell and

tube heat exchanger sealing applications.

The preferred manufacturing approach involved producing a gasket from a wire strip;

thus offering both maximum manufacturing flexibility and cost savings with regard to

material utilisation. The strip is spirally wound however, the strip profile and thickness

differs significantly from that used in the manufacture of SWG. See figure 7. The wire

strip used in the manufacture of the Change gasket approximately 5 times thicker

than that used in the production of SWG. As a consequence significant modification

was required to standard SWG production machinery to enable manufacture.

Figure 7: Gasket Wire Profiles

Upon winding, the unique profile and increased thickness of the wire results in a stiff

yet resilient interlocked structure with greatly improved handling characteristics

compared to that of standard SWG. The resulting stiffer construction can in many

instances negate the requirement to control gasket compression allowing its use in

applications where space may be limited. A soft filler material, such as graphite, is

incorporated into the gasket during the winding process. Once wound the resulting

profile of the gasket sealing faces bears resemblance to that of a kammprofile

gasket. Both sealing faces may be subsequently covered with a layer of soft facing

material, providing low stress sealing and good conformance on poor flange faces. A

schematic of a sectioned profile of the gasket in given in figure 8.

ChangeSpiral Wound



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Figure 8: Change Gasket - Section

One of the main operational challenges in manufacturing the gasket was how to

permanently fix the winding strip at the start and termination of the winding process.

Conventional resistance spot welding could not be used because of the increasedwire thickness. Weld integrity in an unconfined spiral wound seal arrangement such

as this plays a crucial role in maintaining seal integrity. To this end various welding

techniques were investigated. Following extensive trials, high power, precision laser

welding was selected. Laser welding involves creating a localised, high precision,

deeply penetrating high energy plasma reducing. Due to the localised nature of the

technique thermal stresses are kept to a minimum resulting in a reduced heat

affected zone compared with traditional welding methods. No additional welding

material is involved, the result is a high strength weld capable of maintaining gasket

integrity when exposed to fluctuating high load. See figure 9 for weld detail.

Figure 9: Change Gasket Weld Detail

4.2 Laboratory Testing

During the development programme the Change gasket was subjected to the usual

battery of both standard and non-standard laboratory testing. ASME VIII (ref.[2],

PVRC ROTT (ref. [3]) and EN13555 (ref. [4]) testing was undertaken to assess

sealing behaviour under assembly and dynamic loading conditions and to generate

the relevant gasket constants. However, to assess long term multi-cyclic load

SerrationsMetallic winding strip

Section View Plan View



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conditions and attempt to accurately simulate shell and tube heat exchanger

conditions, particularly at the tube sheet gasket positions, testing was undertaken

using a modified extended version of the Shell Thermal Cycle test. This test is part ofthe Shell Type Approval Testing requirement for semi-metallic gaskets. In addition to

testing the Change gasket benchmark testing was also undertaken on all the

previously reviewed gasket styles.

4.2.1 Extended Shell Thermal Cycle Testing

The Shell thermal cycle test involves subjecting a standard gasketed bolted

connection to a series of thermal cycles. Leakage performance is determined by a

series of pressure decay tests carried out at defined stages during the test.

The test rig consists of a pair of ASME B16.5 NB 4”raised face flanges incorporated

into a welded pressure vessel that is fitted with an internal heating element. The test

gasket is stressed via hydraulic tensioning of eight ASTM A193 B16 stud bolts. The

induced bolt stress is 290 N/mm2. The test medium is nitrogen. A schematic of the

test rig is given in figure 10.

The test regime was as follows:

a) Assemble and pressurise the test rig to 51 bar

b) Undertake room temperature pressure decay test over 1 hourc) Depressurise and heat up the test rig to 320

oC at 2

oC/min.

d) Re-pressurise to 33 bar followed by pressure decay test over 1 hour

e) Allow to cool

In the standard test parts c) through e) are repeated a total of 3 times. Test pass or

fail is determined by pressure testing; the maximum allowable pressure drop either

prior to and/or after the final thermal cycle is 1 bar (14.5 psi) over a one hour period.

Leakage rates higher than this result in test failure. Each thermal cycle takes

approximately 24 hours to complete. Extended benchmark testing of the all the

gasket styles involved increasing the number of thermal cycles from three to twentyfour or until test failure. The number of cycles and test temperature was selected

after consultation with industry engineers; it being regarded as being representative

of a typical process temperature and number of process trips, or thermal excursions

an exchanger may be subjected to between scheduled maintenance outages.

Graphical test results are shown in figure 11. Under standard Shell test conditions

i.e. three thermal cycles, all gasket styles are compliant. However test modification to

increase the number of thermal cycles highlighted some significant differences in

sealing performance.



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Figure 10: Shell Test Rig – Schematic

4.2.2 Extended Thermal Shell Cycling - Test Results

With the exception of the corrugated metal gasket and the Change gasket all othergasket styles; kammprofile, double jacketed gasket and SWG gave similar levels of

performance, all failing to meet or exceed the maximum allowable pressure drop

requirement of 1 bar/hour after exposure to between 14 to 16 thermal cycles. The

corrugated metal style gasket performed the worst failing pressure test after 5

thermal cycles.

It can be seen from the data that the Change gasket significantly out performed all

other gasket styles and was the only gasket able to meet the requirements of the

test. After the completion of all 24 cycles the measured pressure drop was 0.07 bar

(1 psi). The test was subsequently repeated and the results proved to be consistent.

The test data clearly demonstrates that, under the described test regime, the Change

gasket offers significantly improved sealing performance under thermal cycling

conditions compared to traditional gasket styles.

Test gasket

Heating element

Thermal insulation

Pressure isolation valve

To vent/gas supply



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Figure 11: Pressure v Thermal Cycle Number

Legend:

DJ = Double jacketed gasket

CMG = Corrugated metal gasket

CGI = Spiral wound gasket

Kamm = Kammprofile gasket

Change = Change gasket

Red broken line ( ) = Failure point (1 bar/hr pressure drop)

4.2.3 Radial Shear Testing

An important consideration when designing a gasket for heat exchanger applications

is the possibly of the gasket being exposed to radial shear. This can occur due to

differences in radial expansion when the tube sheet is exposed to a thermal gradient

resulting from different temperatures on the shell and tube-side of the exchanger.

Radial shear can result in gasket damage and seal failure. Work carried out in thepast has shown that certain gasket styles are superior to others at resisting radial

shear. See ref [5].

A test protocol has been developed by Yarmouth Research in the US, see ref. [6].

The test involves exposing a gasket located in a tongue and groove flange

arrangement to a thermal differential induced by heating one of the flanges to 300oC

while cooling the other using water. The minimum amount of radial shear the test

gasket is exposed to during the test is 0.51 mm. The test is comprised of exposing a



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test gasket to 20, 40, 60, 80 and a 100 thermal cycles. Following each radial shear

test the assembly is pressure tested at room temperature with nitrogen at 40 bar on

an hourly basis over the next 4 hours. In addition to leakage performance boltrelaxation is recorded at the start and finish of each test. This is an important

indicator of seal performance as loss of strain energy in the connection is primary

reason for seal failure. SWG have been shown to perform well in radial shear testing

and they typically give rise to an average bolt relaxation value in the order of 25%.

Average bolt relaxation as measured for the Change gasket was 15% with no

significant leakage observed, see ref. [6].

4.2.4 Sealing on Nubbins

Depending on the age and original gasket style designed for the heat exchanger it isnot uncommon to for girth flanges to have stress raising nubbins incorporated into

the bottom of the recess of the mating flange. See figure 12. The primary objective is

to raise the unit stress on the gasket thus improving sealing efficiency on assembly.

Nubbins raised sections in the bottom of the flange, are generally used in

connections where solid metal or double jacketed gaskets are used. Their presence

can preclude the use of SWG and kammprofile sealing technology as they can result

in gasket damage that can compromise seal integrity. As a consequence nubbin

removal is normally recommended when using SWG or kammprofile gaskets, adding

additional cost and time to maintenance programmes.

Figure 12: Flange and Gasket Arrangement with Nubbin - Section

Sealing performance of the Change gasket has been assessed in a flanged

connection with nubbins present. A specially developed test fixture was used in

which a 0.4mm x 3mm (high x wide) nubbin was machined. A test gasket was

installed and the test fixture pressurised using nitrogen gas, across a range

pressures from 17 to 103 bar (250 to 1500 psi). The test pressure was held for 5

minutes during which the test rig was submerged in water and subjected to a bubble

test. Leakage performance at two gasket stresses was investigated. The tabulated

test data is shown in table 1. Post test visual inspection of the sectioned gasket was

carried to assess gasket damage. The gasket remained intact with only minor

Recessed flange

Spigot flange

Gasket

Nubbin



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deformation. Several Change gaskets have now been successfully used in

connections in which stress raising nubbins are present.

Table 1: Change Gasket Leakage Performance with Nubbins

Internal Pressure N2(bar) Leak rate (bubbles/min)@

Gasket stress 103 N/mm2

Leak rate (bubbles/min)@

Gasket stress 206 N/mm2

17.2 0 0

34.5 0 1

51.7 0 1

68.8 0 0

86.0 0 0

103.4 0 0

5 Field Service

Since the development and introduction of the Change gasket many have been

supplied and are currently being used in non-standard connections in shell and tube

heat exchangers around the world.

The following is an account of the Change gasket being used in a problematic

application at Total’s Lindsey Oil Refinery (LOR) located at Immingham in the UK.

5.1 Background

Flexitallic Application Engineers were consulted following repeated leakage issues

on two banks of shell and tube heat exchangers on the visbreaker unit. The

visbreaking process is designed to reduce the viscosity and pour point of the heavier

hydrocarbon fractions resulting from vacuum distillation (VD) of crude oil.

Visbreaking is one of several cracking processes used on modern complex oil

refineries, see ref. [6]. The commercial success of any refining operation depends to

a great extent on the ability of the refinery to process a range of crude oil qualities

and ensure optimum product yield is aligned with market demand. This generally

means having the flexibility to convert higher molecular weight hydrocarbon fractions

to lower weight fractions with the correct molecular structure. Visbreaking is a

relatively mild thermal cracking procedure compared to other commonly usedprocesses. Mild refers to the extent of the cracking reaction rather than the absolute

temperatures at which it takes place. The extent of the reaction is controlled by



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quenching the products immediately after leaving the reactor. Two technologies are

commonly employed, furnace (or coil) and soaker (or drum) processes. In the former

the visbreaking reaction takes place directly in the furnace coils while in the latter itoccurs away from the furnace in a specially designed unit known as a soaker.

Soaker units operate at relatively low temperatures and use longer residence times

compared with coil units. A simplified schematic of the soaker visbreaking process is

shown in figure 13. The main benefits of the soaker process are reduced energy

requirements and increased run times because of reduced reactor coking rates

however, the decoking process itself can be more complex due to soaker design.

Coking, or fouling, is an important consideration in refinery processes as it can have

a significant impact on plant efficiency. Carbon deposits have a negative effect on

both heat transfer efficiency and feedstock and product throughput.

Figure 13: Soaker Visbreaking Process - Simplified Schematic

5.2 Application Details

The visbreaking operation used on the refinery is based on soaker technology. Prior

to visbreaking the virgin feedstock (VD residues) is pre-heated via a bank of four

shell and tube heat exchangers. The exchangers were commissioned around 33

years ago and have history of ‘managed leakage’. Over the years the girth flanges

have been re-machined to rectify flange distortion and corrosion issues and remain

within corrosion allowance limits. Mechanical clamps and injectable sealants have

been employed on a temporary basis on a number of occasions to mitigate leakage

at the tube-sheet connections and allow continued running of the visbreaker unit in

between planned shutdown programmes. Several different gasket styles have been

previously evaluated with some noted differences in performance, however, none

have given the required level of service.



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The exchangers have been designed to ASME VIII Div. 1 TEMA R pressure vessel

code and categorised as TEMA Class AES. The flanges do not contain stress raising

nubbins. Test, design and operating conditions are given in table 2.

Table 2: Test, Design and Operating Conditions

Shell Side Tube Side

Design Pressure (barg) 38 36

Test Pressure (barg) 57 56

Design Temperature (oC) 370 390

Op. Temp Outlet (oC) 335 283

Op. Temp Inlet (oC) 192 365

Media VD Residue Visbreaker Residue

As is common, for reasons of economy visbreaker residue is used as the heating

medium on the tube-side of the exchanger. The negative impact of this is regular

coking or fouling of the internals of the exchanger bundle.

Two banks of exchangers are used to pre-heat feedstock, however only one bank is

in operation at any one time. This allows routine maintenance to be carried out

without shutting down the visbreaker unit. Each bank of 4 heat exchangers consists

of 2 x 2 vertically aligned units designated 91E1 - A, B, E, F and 91E1 - C, D, G, H.

Each of the two banks is connected in series. Photographs of the units in operation

are given in figure 14.



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Figure 14 Visbreaker Preheater Shell and Tube Heat Exchangers

Because of the nature of the feedstock and pre-heat medium (visbreaker residues)

fouling or coking of the exchangers, particularly on the tube side is commonplace.

For operational efficiency reasons cleaning or de-coking is undertaken on a regular

basis. Cleaning requires taking the units off line. The cleaning regime employed

depends on the extent and location of coking. If coking is limited to the tube-side of

the exchanger cleaning involves removal of the channel cover and ‘reverse jet’cleaning the tube bundle using ultra-high pressure water. In this instance the tube

sheet gaskets are not removed and remain under compressive load. However the

act of taking the unit off line subjects the tube sheet gaskets to significant thermal

shock. Subsequently some time in the future; typically after a 2 to 3 month period the

tube sheet gaskets are subject to additional thermal shock when bringing the unit

back on line and up to operational temperature. Coking on the shell side of the

exchanger is less problematic and requires less frequent attention however, shell

side cleaning requires the removal of the shell cover i.e. the tube bundle is fully

removed, and removal of the floating head to allow full ‘through jet’ cleaning. In thiscase the connection across the tube-sheet is broken and new gaskets installed on

re-assembly of the exchanger. The frequency of cleaning (de-coking) is primarily

determined by the nature of the VD feedstock (crude) and process conditions. Over

the last few years and increasingly as we go into the future the ability of a refinery to

process a wide range of heavier and dirtier crude feed stocks is a key factor to

economic success. However as can be seen in this example it can place increasing

demands on gasketed flanged connections.

Changes in product demand patterns have also, over the last few years exacerbated

sealing issues on the pre-heater exchangers. The need for bitumen a key raw

material used in the production of road surfaces has increased and, as consequence

has become an economically viable refinery product. This has had a direct impact on



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the production of visbroken distillates as VD heavies are an important source of

bitumen. The demand pattern for bitumen is seasonal and summer months in Europe

generally gives rise to an increase in demand, providing economic incentive to

maximise the production of VD heavies. At such times the visbreaker unit is in put

into bypass mode reducing the inlet and outlet temperatures on both the shell and

tube side of the exchanger in the order of 100oC. The frequency of placing the

visbreaker unit to bypass mode depends on product demand patterns. Typically the

exchangers are in bypass mode 12 days per calendar month during summer. This of

course leads to additional non-cleaning related thermal transients resulting in

additional stress fluctuations across the tube-sheet connection.

It estimated that the average number of thermal cycles the exchangers are exposed

to because of cleaning and/or standby mode operation is two per month.

5.3 Gasket Selection

Over recent years many different gasket styles have been evaluated. Originally the

exchangers were supplied with double metal jacketed gaskets. Following leakage

issues spiral wound gaskets were installed and more recently kammprofile gaskets

have been used. Both spiral wound and kammprofile gaskets proved to offer

advantages over jacketed gaskets however all gasket styles failed with regard to

providing long term i.e. greater than shell side cleaning interval requirements, leak

free performance at the tube sheet gasket locations.

In light of gasket history and operational considerations Change gaskets were

designed for both the channel/tube sheet and shell/tube sheet locations. Because of

the relatively large size of the connection, in excess of 1.5 m diameter and the

available space both gaskets were fitted with solid inner rings. The channel tube

sheet gasket was fabricated with internally welded double jacketed pass bars. See

figure 15 for gasket details.

Figure 15: Change Gasket Channel

to Tube Sheet



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Gasket materials of construction were as follows:

• Winding, inner ring and jacket material: UNS31603 (SS316L, WN1.4404)

• Filler and facing material: High purity graphite (Flexicarb HT)

Bolting and tightness calculations were carried out in accordance with ASME VIII

Appendix II and PVRC Convenient method (Draft). ASME VIII appendix II

calculations indicated code compliance with operational conditions requiring a bolt

area of 47076 mm2 (Am1) with a corresponding available bolt area (Ab) of 54387

mm2. PVRC Convenient calculations indicated a T3 tightness class at a residual bolt

stress of 210 MPa (30.5 ksi) with an equivalent gasket stress of 110 MPa (15.9 ksi).

The gaskets were installed using best practice assembly techniques. Hydraulic

torque was used to induce bolt stress during flange assembly. Target bolt stress oninstallation was 310 MPa (45 ksi) with an equivalent assembly gasket stress of 162

MPa (23.5 ksi). Flange calculations were carried out to ensure flange stresses and

bending moments were kept within acceptable ASME/Taylor Forge design limits.

5.4 Sealing Performance

At the writing of this paper the Change gaskets were installed 14 months ago (May

2013) on units 91-E1 A, B, E and F and 8 months ago (October 2013) on exchangers

91-E1 C, D, G and H. Throughout these periods the visbreaker preheaters have

been in constant use; processing visbreaker feed stock or in standby mode,depending on product demand profile. Since installation there has been no

requirement to undertake shell side cleaning so the tube-sheet connection has not

been opened and the gaskets have not been replaced. However, tube side cleaning

has been carried out on a number of occasions. The total number of thermal cycles

to which the tube-sheet connection has been subjected is in the order of 28 for units

A through D and 16 for units F through G. Regular visual monitoring of the

connections has shown no incidence of leakage across any of the 16, previously

problematic tube-sheet connections. Discussions with plant engineering personnel

suggest that if using conventional gasket technology that an incidence of leakagewould have been extremely likely over this time period and that the adoption of

Change gasket technology represents a significant improvement in the maintenance

of joint integrity on these problematic units.



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6 Conclusions

Under the refinery field conditions described the Change gasket offers a significant

advantage over traditional gasket designs namely; SWG, jacketed and kammprofilegaskets, in heat exchanger tube-sheet connections subject to thermal transients.

Under the laboratory test conditions described the Change gasket is able to

accommodate fluctuations in both axial and radial load to a much greater degree,

maintaining a high integrity seal when benchmark tested against other gasket styles.

The Change gasket represents a fundamentally new design concept offering the

optimum balance of resilience; usually associated with spiral wound gaskets and

stiffness; usually associated with kammprofile gaskets, over the typical stress

range(s) generated in bolted connections.

7 References

[1] Standards of the Tubular Exchanger Manufacturers Association (TEMA) 9th

Edition (2007).

[2] ASME VIII Div 1 (2010) Appendix II Rules for bolted flanged connections with

ring type gaskets

[3] PVRC ROTT Draft 10.01 Standard test method for gaskets constants for

bolted joint design (April 2001)

[4] BS EN13555 2014 Flanges and their Joints. Gasket Parameters and test

procedures relevant to the design rules for gasketed circular flange

connections.

[5] Heat exchanger gaskets radial shear testing. Viega, Kavanagh and Reeves.

PVP2008-61121

[6] Yarmouth Research and Technology. Report/Project No. 2120450 2013

[7] William L Leffler. “Petroleum Refining in Nontechnical Language” 4th Edition

(2008).

[8] Russ Currie “Change Gasket – Overview and Recent Developments”

PVP2013-97050 ASME Pressure Vessels and Piping Conference. Paris 2013

8 Acknowledgements

I would like to express my special thanks to Mr Kevin Wallace, plant engineer on

Totals Lindsey Oil Refinery for his support in producing this paper.

Refinery Based Case Study of a Novel Gasket Designed for Use in Problematic Shell and Tube Heat...

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